215anotes_final

215anotes_final - EE215A B. Razavi Fall 08 HO #11 Why...

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Unformatted text preview: EE215A B. Razavi Fall 08 HO #11 Why Analog?- Naturally-occurring signals, e.g., voice and video, are analog. - System and medium non-idealities often make it necessary to treat digital signals as analog: o Data Retrieved from Disk o Digital Wireless Communications Why is analog design such a big deal?- Entails more trade-offs than digital design: Digital Analog speed power speed power- More sensitive to noise and cross-talk.- More sensitive to second-order effects in devices.- More difficult to automate.- More difficult to model and simulate.- We want to design analog circuits in mainstream VLSI technologies, e.g., CMOS, with no additional processing steps, trimming, factory calibration, etc. 1 EE215A B. Razavi Fall 08 HO #11 REVIEW OF MOS DEVICES MOS Structure (NMOS)- A piece of polysilicon with a width of W and length of L on top of a thin layer of oxide defines the gate area.- Source and drain areas are heavily doped.- Substrate usually tied to the most negative voltage.- Le ff = L 2L D , where L D is the side diffusion of source and drain. MOS Symbols (Enhancement Type) PMOS NMOS - MOS structure is symmetric.- MOS devices have a very high input impedance. 2 EE215A B. Razavi Fall 08 HO #11 MOS characteristics o How does the device turn on and off? o What is the drain-source current when the device is on? Threshold Voltage For V GS < V TH , holes in substrate are repelled from gate area, leaving negative ions behind. (No current flows because no carriers are available.) A depletion region forms under the gate. For V GS V TH , electrons are attracted to the interface under gate, establishing a channel for conduction. The channel is also called the inversion layer. For V GS V TH , depletion region under channel remains relatively constant, but the charge in inversion layer increases . Turn-on process not really abrupt, i.e., for V GS V TH , I D > o. => Sub- threshold conduction (considered later). For a long-channel device with uniform substrate doping: (and V sub = V source ): 3 EE215A B. Razavi Fall 08 HO #11 where Often need to implant the channel to obtain the right threshold. A helpful approximation: For V GS V TH , there is only depletion region in the gate area; for V GS V TH , the depletion region is constant and the inversion layer charge increases. A useful Lemma: If a conductor carries a constant current I and it has a charge density (charge per unit length ) of Q d and the charge moves with a velocity v: I = Q d v MOS I V Characteristics For V DS > o, the inversion layer charge is non-uniform: 4 EE215A B. Razavi Fall 08 HO #11 Note that as we approach the end of the channel, the charge density falls....
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This note was uploaded on 06/18/2010 for the course CE 01 taught by Professor Bazgei during the Spring '09 term at UCSC.

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215anotes_final - EE215A B. Razavi Fall 08 HO #11 Why...

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